During the exploration for fossil fuels, the drill will sometimes encounter a pocket or reservoir of high pressure (e.g., several thousand pounds per square inch (psi)) subterranean gas (such as steam, sulfite gas, natural gas or the like) which is then free to rise through the well casing. Of course, the uncontrolled release of such high pressure gas at the well head could cause severe damage to equipment and potential injury to the drilling crew. It is therefore typical for the well head to be provided with apparatus known in art-parlance as a blowout preventer (BOP) which minimizes the risk of such uncontrolled escape of high pressure gas.
Conventional BOP'S, however, are extremely large and cumbersome valving devices which are positioned above-ground directly on the well head. Furthermore, while conventional BOP's have a central bore to allow for passage of the drill stem during normal drilling operations, the drill stem must usually be physically stopped in the well casing when a pocket of high pressure subterranean gas is encountered. Stoppage of the drill stem within conventional BOP's allows the drilling crew to pump a viscous drilling fluid (known as "drilling mud") into the well, in addition to allowing the conventional BOP to close and function in its intended manner.
Once the gas has been effectively trapped at the top of the well casing between the BOP and the viscous drilling mud, the gas will then controllably be evacuated from the well head by an above-ground valving manifold (e.g., a so-called "choke-and-kill" manifold). The valving manifold includes several actuable valves which can be opened selectively so as to bleed the trapped high pressure gas from the top of the well casing in a controlled manner. Once the pressure within the top of the well casing is reduced to within acceptable limits, the BOP may again be opened, the drill stem reinserted into the well, and the drilling operation resumed.
Of course, it is quite time consuming (and thereby costly) to stop the drilling operation to allow for removal of high pressure gas each time a high pressure gas pocket is encountered. Additional significant costs are incurred in subterranean drilling for fossil fuel due to the relatively massive size of conventional BOP's and the auxiliary handling equipment (e.g., drilling rigs, hoists, heavy-duty motors and the like) needed to position the conventional BOP on the well head, and remove it for servicing/inspection. It would therefore be desirable if a reduced-size BOP was provided which could statically reduce the pressure of rising high pressure subterranean gases without necessarily requiring stoppage of the drilling operation. It is towards providing such a BOP that the present invention is directed.
Broadly, the blowout preventer (BOP) according to this invention is embodied in a tubular shell which is insertable within, and thus contained by, an upper portion of the well casing. The tubular shell includes a removable inner sealing/pressure-reducing assembly which provides for a high-pressure seal against the exterior surface of the drill stem as well as reducing the pressure of rising subterranean gas. In this regard, the sealing/pressure-reducing assembly includes a channel passageway for the rising subterranean gas. Most preferably, the channel passageway is in the form of a segment of a right cylinder which is established collectively by a longitudinal side wall of the sealing/pressure-reducing assembly and an interior cylindrical surface of the tubular shell within which the sealing/pressure-reducing assembly is seated. A series of orificed baffles is disposed transversely within the channel passageway with the individual baffles being longitudinally spaced-apart from one another so as to define pressure-reducing chambers between adjacent baffle pairs. In this manner, the pressure of the subterranean gas will be reduced as the gas flows upwardly through the channel.
A longitudinally separated pair of split sealing blocks is also preferably disposed within the sealing/pressure-reducing assembly. The sealing blocks are mounted for reciprocal lateral movements towards and away from the central bore of the BOP according to this invention so as to be capable of movements into and out of sealing relationship with an exterior surface of the well stem. A rotary actuator system operatively coacts with the sealing blocks in such a manner that rotary motion of the actuator system is controllably translated into reciprocal lateral movements of the sealing blocks.
The BOP according to this invention is thus capable of providing a high-pressure sliding seal with the exterior surface of the drill stem while also allowing substantial pressure reduction of the rising subterranean gas to occur statically by means of the static orificed baffles disposed in the channel passageway. In this manner, the dangers associated with the uncontrolled escape of high-pressure subterranean gas is significantly reduced (if not virtually eliminated) by the BOP of this invention while avoiding necessary stoppage of the drilling operation during such gas pressure reduction.
Further aspects and advantages of this invention will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.